This invention relates to the preparation of anilines, more particularly, to a process for preparing an aniline at high conversion and high selectivity by reacting a phenol with an amination agent in the presence of a specific catalyst while maintaining the catalytic activity for an extended period of time.
Anilines are a group of chemicals of industrial importance finding use as rubber vulcanization promotors, dyes, mordants, medicines, explosive stock, and starting material for diphenylmethane diisocyanate (MDI). Aniline derivatives such as toluidine, cumidine, methylcumidine, and xylidine recently find increasing utility as starting materials for photographic agents, agricultural agents, and medicines.
These anilines are produced in the prior art by (1) reducing aromatic nitro compounds with hydrogen, (2) reacting halogenated aromatic compounds with aqueous ammonia at elevated temperatures under a certain pressure, and (3) reacting phenols with ammonia.
The first method (1) using aromatic nitro compounds requires a great quantity of sulfuric acid or nitric acid as the agent for nitrating aromatic compounds, which is followed by a neutralizing step requiring a great quantity of alkali such as sodium hydroxide. There is yielded water containing salts in high concentrations. In addition, as indicated in Japanese Patent Application Kokai No. 48-67229, nitrogen oxide gases evolve during the step of forming nitro compounds, causing air pollution. In nitrating alkylphenols, there are produced a variety of isomeric by-products in addition to the desired nitro compound. These isomers are difficult to isolate. It is thus difficult to produce pure anilines in high yields.
The second method (2) using halogenated aromatic compounds has a critical problem that an expensive corrosion-resistant equipment must be installed for the preparation of halogenated aromatic compounds because very highly corrosive chlorine is used therefor. It is known that reaction between chlorobenzene and ammonia results in low yields despite elevated temperature and pressure. This method is seldom utilized in the art except when the halogenated aromatic compound is p-nitrochlorobenzene having a nitro group at the para-position of chlorobenzene.
In contrast to methods (1) and (2), the third method (3) based on reaction of phenols with ammonia currently draws attention and becomes a mainstream process for the preparation of anilines. This method enables to produce anilines merely by passing phenols and ammonia through a fixed bed of catalyst. This method has many advantages because it is a very simple process which does not yield nitrogen oxide gases causing air pollution or a great amount of water.
A typical method for the preparation of anilines by reaction of phenols with ammonia is disclosed in Japanese Patent Publication No. 42-23571. According to the disclosed method, aminated benzenes such as aniline are prepared by reacting a hydroxybenzene such as phenol with an amination agent in the presence of a catalyst selected from the group consisting of silica-alumina, zirconia-alumina, titaniaalumina, zirconia-silica phosphates and tungstates at a temperature of 300.degree. to 600.degree. C. This publication teaches that weakly acidic solid acids such as .gamma.-alumina catalysts are insufficient because of low activity, but a silica-alumina catalyst which is a strongly acidic solid acid is fully effective as the catalyst for such amination reaction.
The use of strongly acidic solid acid catalysts such as silica-alumina catalyst, however, undesirably invites side reactions including decomposition of anilines and formation of resinous by-products although these acid catalysts have high initial activity for amination reaction. These catalysts have a critical problem that once such a resinous substance is deposited on the catalyst surface, the catalyst undergoes rapid deactivation because active sites are covered with the resinous deposit. Thus the catalyst must be frequently regenerated.
One attempt to solve these problems is proposed in Japanese Patent Application Kokai No. 48-67229, in which reaction of phenol with an amination agent is carried out using a catalyst having a lower acid strength than the above-mentioned silica-alumina catalyst (pKa&lt;-8.0), that is, titania-zirconia and titania-silica catalysts which are solid acid catalysts having an acid strength in the range of pKa-5.6 to -3.0. Even with the use of such catalysts, the reaction temperature must be increased to as high as 400.degree. to 500.degree. C. in order to accomplish effective amination reaction. The elevated reaction temperature accelerates decomposition of the amination agent or ammonia, producing nitrogen according to the following scheme: EQU NH.sub.3 .fwdarw.1/2N.sub.2 +3/2H.sub.2
The reactor undergoes embrittlement with nascent nitrogen. Undesirably, the effective life of the reactor is significantly reduced. It is also observed that the catalyst suddenly loses its activity within a time as short as about 40 hours. For these reasons, this method is very difficult to commercially practice.
Japanese Patent Application Kokai No. 46-23052 discloses amination of phenols using a combined catalyst comprising a dehydrating solid acid catalyst combined with a hydrogenating catalyst. Japanese Patent Application Kokai No. 46-23053 discloses amination of phenols using a catalyst comprising alumina or silica combined with an oxide selected from magnesia, boria and thoria. In either case, the activity sustaining time is improved to only 50 to 100 hours, leaving the catalyst deactivation problem unsolved.
In summary, the prior art known methods for preparing anilines by amination of phenols require high temperatures of at least 400.degree. C. in order to accomplish effective amination reaction, which in turn, induces decomposition of the amination agent or ammonia, causing embrittlement of the reactor with nascent nitrogen. These methods also suffer from the critical problem of frequent catalyst regeneration because the catalytic activity is shortly lost due to contamination of the catalyst surface by a resinous substance resulting from decomposition of anilines or coverage of the catalyst surface with a carbonaceous deposit resulting from decomposition of an organic substance.